In a wireless communication system such as a wireless LAN (Local Area Network), a wireless terminal establishes wireless communication with a single base station and communicates with other wireless terminals via the base station.
If transmission strength between the wireless terminal and the base station weakens during wireless communication due to movement or the like, the wireless terminal switches the wireless connection target to another base station that is located in the vicinity of the base station and has stronger transmission strength, thereby performing so-called “handover”.
Hereinafter, the wireless terminal is referred to as a station, and the base station is referred to as an access point (or abbreviated AP). In particular, an access point with which the station has established a wireless connection (handover source) is called a serving access point, and an access point that is located in the vicinity of the serving access point and with which the wireless terminal has not established a wireless connection (handover target candidate) is called a neighboring access point.
A terminal such as a PDA (Personal Digital Assistant) or a PHS (Personal Handy-phone System) including a wireless LAN function may be used as the wireless terminal.
Generally speaking, when the transmission strength (for example, a Received Signal Strength Indication (RSSI)) between the station and the serving access point falls below a predetermined threshold, the station judges that handover is necessary. To search for an access point as a handover target, the station performs a search, called a scan, for connectable neighboring access points. During the scan, the station transmits an access point search signal called a probe request frame to neighboring access points whose used channels are different, and receives a response signal called a probe response frame that is sent from each neighboring access point as a response to the signal. The station performs handover upon selecting, from among the neighboring access points, the neighboring access point that has the highest RSSI when the probe response frame is received.
Here, for example, in the known IEEE 802.11b standard, 14 channels in a frequency band from 2.412 GHz to 2.483 GHz are used in Japan. For this reason, during the above-described scan, the station must search for neighboring access points either on all the channels or on selected ones of the 14 channels that are in a predetermined range.
Also, patent document 1 discloses a technology for reducing the number of neighboring access points to be scanned for handover by accumulating a history of a number of times that the station has performed handover to each of the neighboring access points, and targeting a predetermined number of neighboring access points for scanning whose history indicates a high number of times that handover has been performed.    Patent document 1: Japanese Patent Publication No. 3636696